Ökad växthuseffekt har länge varit i fokus för dess inverkan på framtida klimat. Det är i huvudsak mänskliga utsläpp som är orsaken till ökade mängder växthusgaser i atmosfären och stora ansträngningar görs för att utsläppen på sikt ska minska. I klimatmodeller beskrivs växthusgasbalansen utifrån både mänsklig och naturlig påverkan. Förståelsen för naturlig påverkan har länge varit begränsad och mer forskning behövs inom området. Flera studier visar på att sötvattensystem (sjöar, vattendrag osv.) avger växthusgaser som koldioxid, CO2 och metan, CH4, i större proportioner än vad som tidigare varit känt. Denna studie syftade till att undersöka CO2 och CH4-flöden från Erssjön i Skogaryd, Västergötland (en typisk svensk skogssjö) och få förståelse för hur den omgivande skogen påverkar såväl flöden som turbulens (utbyteskoefficienter) över sjön. Dessa resultat skulle jämföras med tidigare studier från den betydligt större sjön Tämnaren i norra Uppland. Resultaten visar på ett upptag av CO2 som en följd av fotosyntes hos skogen under dagtid, vilket leder till slutsatsen att omgivande skog påverkar växthusgasflödena. En ökning av både CO2 och CH4-flöden över sjön var tydlig nattetid, vilket enligt tidigare studier beror på konvektion i vattnet (liknande resultat för Tämnaren). Ökningen är speciellt utmärkande för CH4-flöden, vilket tros bero på att konvektionen ökar omblandningen och därmed löser upp sedimenterat CH4 och inducerar flerCH4-bubblor. Det sistnämnda kan styrkas med denna studie eftersom svaga vindar (1-2 m/s) varit dominerande i nattmätningarna och därmed har vindpåverkan på flödena varit låg. Resultaten visade likt Tämnaren att omgivningen påverkar turbulensen över sjön. Turbulensen uppstår troligen till följd av friktion vid skogskanten snarare än inverkan från hela skogsomgivningen, då beräknade utbyteskoefficienter inte visade på några större skillnader i spridning när endast data från sjön fanns representerade. Sammanfattningsvis är det intressant att i vidarestudier göra turbulensmätningar runt sjön för att sedan undersöka eventuella samband med utbyteskoefficienter och växthusgasflöden. / Increasing amounts greenhouse gases and its impacts on future climate has been in focus quite some time. The increase is mainly due human sources, and hugh efforts to decrease the emissions are made continuously. Climate models describe the greenhouse gas balance including both human and natural sources. In contrast to human sources, knowledge about natural sources is limited and requires further investigations. Recent studies show that greenhouse gases, such as Carbon dioxid, CO2, and methane, CH4, are emitted from freshwater systems (lakes, rivers etc.) in much larger proportions than what has been estimated earlier. The purpose of this study was to analyze CO2 and CH4 fluxes from a small lake with forest surroundings in southwestern Sweden, and also investigate if the forest affects both fluxes and turbulence (exchange coefficients) above the lake. The results from this study were to be compared with studies of a much larger lake (in eastern Sweden) but with similar surroundings. Results from the present study show an uptake of CO2 due to photosynthesis in the forest during daytime, leading to the conclusion that the surrounding forest affects the greenhouse gas fluxes. Both CO2 and CH4 fluxes increased above the lake during nighttime. According to similar studies (e.g. the larger lake in eastern Sweden), increased nighttime fluxes is an effect of increased convection in the water. The diurnal differences is most distinctive for CH4 fluxes. As suggested in earlier studies, the convection increases mixing in the water, dissoliving sedimented CH4 and inducing ebullition. In this study, low windspeed (1-2 m/s) dominated the nighttime measurements suggesting that ebullition is the main source of increased CH4 fluxes since wind-effects on the fluxes were low. Like the larger lake in eastern Sweden, results in this study indicate that the surroundings affect the turbulence above the lake. A large distribution in the calculated exchange coefficients that doesn’t decrease when only data above the lake is represented, suggests that turbulence above the lake is induced by friction at the forest edge, rather than the whole forest surroundings. Additional turbulence measurements around the lake for comparing with exchange coefficients and greenhouse gas fluxes could be of interest for future studies.

While it is not the most abundant greenhouse gas, a significant portion of the greenhouse effect is caused by methane. The amount of methane in the atmosphere is increasing, indicating that there is a continuous source of methane to the atmosphere. One significant source of methane is freshwater lakes, even though they cover only a small portion of the Earth’s surface. Because of this, it is important to monitor methane fluxes from lakes in order to understand the processes which affect the magnitude of these fluxes. Methane is produced in the sediment at the bottom of the lake, and transported through the water by ebullition, diffusive flux, storage flux, or plant mediated emission. This study looked to examine the amount of methane transmitted to the atmosphere by these processes on Lake Erken in eastern Sweden. Using the eddy covariance method, we can study the methane flux with good spatial and temporal resolution. Regular sampling of lake water, both from the surface and depths of 5 and 10 meters, also helps us to understand the amount of methane dissolved in the lake. These measurements can help us to better understand the transfer velocity, or the efficiency of the exchange between water and air, as well as the amount of methane transported from lakes to the atmosphere. Water sampling showed that there is very little variation in methane concentration between different parts of the lake. Concentrations at four surface locations are nearly identical. These surface measurements are also similar to concentrations at different depths. Over time, the concentrations generally stayed the same, with isolated high and low concentration events. The amount of methane emitted by the lake was studied with the lake divided into a shallow water area, and a deep water area. The magnitude of fluxes from both areas was very similar, but the area of shallow water had a higher total flux. The fluxes were well correlated with wind speed; higher fluxes coming during times with higher wind speed. This relates well to the transfer velocity theory, and the bulk flux approximation. However, there was no clear diurnal cycle in methane fluxes. The fluxes during the night were similar to daytime fluxes. Atmospheric pressure also had an impact on fluxes, with greater fluxes coming at times of lower pressure. A large seasonal variation was clear. More methane escaped the water in autumn and winter than in spring or summer. This is due in part to the fluxes from when the lake freezes over/thaws and the water in the lake turns over, bringing methane rich water from the lake’s bottom to the surface. As expected, the waterside concentration of methane also had a strong correlation with the fluxes. The main conclusions of this study are: 1) Methane fluxes are variable with wind speed, waterside concentrations, and the seasons 2) Water depth and diurnal cycles do not affect methane fluxes as strongly. Keywords: methane, transfer velocity, flux, waterside concentration, eddy covariance